Hello Patrick,
A similar behavior is described in
this KB article, in which possible resolutions are provided.
If this is not the case, it should be better if you could prompt the users to provide more details about the browser, its version etc.
Also, some details about the configuration of the editor and the scenario with which the problem is encountered will be useful to recreated the problem and investigate it properly.
Regards,
Ianko
Telerik
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